Recently, there is an increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention.
Secondary batteries are chemical batteries capable of repeated charge and discharge cycles by means of reversible interconversion between chemical energy and electric energy, and may be classified into Ni-MH secondary batteries and lithium secondary batteries. Lithium secondary batteries include secondary lithium metal batteries, secondary lithium ion batteries, secondary lithium polymer batteries, secondary lithium ion polymer batteries, etc.
Because lithium secondary batteries have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries), they are produced commercially by many production companies. However, most lithium secondary batteries have different safety characteristics depending on several factors. Evaluation of and security in safety of batteries are very important matters to be considered. Therefore, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards.
Currently available lithium ion batteries and lithium ion polymer batteries use polyolefin-based separators in order to prevent short circuit between a cathode and an anode. However, because such polyolefin-based separators have a melting point of 200° C. or less, they have a disadvantage in that they can be shrunk or molten to cause a change in volume when the temperature of a battery is increased by internal and/or external factors. Therefore, there is a great possibility of short-circuit between a cathode and an anode caused by shrinking or melting of separators, resulting in accidents such as explosion of a battery caused by emission of electric energy. As a result, it is necessary to provide a separator that does not cause heat shrinking at high temperature.
To solve the above problems related with polyolefin-based separators, many attempts are made to develop an electrolyte using an inorganic material serving as a substitute for a conventional separator. Such electrolytes may be broadly classified into two types. The first type is a solid composite electrolyte obtained by using inorganic particles having lithium ion conductivity alone or by using inorganic particles having lithium ion conductivity mixed with a polymer matrix. See, Japanese Laid-Open Patent No. 2003-022707, [“Solid State Ionics”—vol. 158, n. 3, p. 275, (2003)], [“Journal of Power Sources”—vol. 112, n. 1, p. 209, (2002)], [“Electrochimica Acta”—vol. 48, n. 14, p. 2003, (2003)], etc. However, it is known that such composite electrolytes are not advisable, because they have low ion conductivity compared to liquid electrolytes and the interfacial resistance between the inorganic materials and the polymer is high while they are mixed.
The second type is an electrolyte obtained by mixing inorganic particles having lithium ion conductivity or not with a gel polymer electrolyte formed of a polymer and liquid electrolyte. In this case, inorganic materials are introduced in a relatively small amount compared to the polymer and liquid electrolyte, and thus merely have a supplementary function to assist in lithium ion conduction made by the liquid electrolyte.
As described above, electrolytes according to the prior art using inorganic particles have common problems as follows. First, when liquid electrolyte is not used, the interfacial resistance among inorganic particles and between inorganic particles and polymer excessively increases, resulting in degradation of quality. Next, the above-described electrolytes cannot be easily handled due to the brittleness thereof when an excessive amount of inorganic materials is introduced. Therefore, it is difficult to assemble batteries using such electrolytes. Particularly, most attempts made up to date are for developing an inorganic material-containing composite electrolyte in the form of a free standing film. However, it is practically difficult to apply such electrolyte in batteries due to poor mechanical properties such as high brittleness of the film. Even if the content of inorganic particles is reduced to improve mechanical properties, mixing inorganic particles with a liquid electrolyte causes a significant drop in mechanical properties due to the liquid electrolyte, resulting in a fail in the subsequent assemblage step of batteries. When a liquid electrolyte is injected after assemblage of a battery, dispersion of the electrolyte in a battery needs too long time and actual wettability with electrolyte is poor due to the high content of the polymer in the organic/inorganic composite film. Additionally, addition of inorganic particles for improving safety causes a problem of a significant drop in lithium ion conductivity. Further, because the electrolyte has no pores therein or, if any, has pores with a size of several angstroms (Å) and low porosity, the electrolyte cannot sufficiently serve as separator.
In addition, U.S. Pat. No. 6,432,586 discloses a composite film comprising a polyolefin-based separator coated with silica, etc., so as to improve the mechanical properties such as brittleness of the organic/inorganic composite electrolyte. However, because such films still use a polyolefin-based separator, they have a disadvantage in that it is not possible to obtain a significant improvement in safety including prevention of heat shrinking at high temperature.